1. Field of Invention
The invention relates to a motor.
2. Description of Related Art
Conventionally, in a motor utilizing magnet torque and reluctance torque together, for example in a permanent magnet synchronous motor, the permanent magnet for generating magnet torque is disposed on a d axis of a rotor, and a projecting pole for generating reluctance torque is disposed on a q axis. The magnet torque and reluctance torque are changed by changing a current phase indicating the position of an electromagnet formed by a current supplied to a stator coil.
FIG. 2 is a relation drawing of the current phase and torque. In this figure, the current phase is plotted on the horizontal axis, and torque on the vertical axis. In this figure, Tm indicates magnet torque, Tr indicates reluctance torque, and Tt indicates a total torque indicating a sum of the magnet torque Tm and the reluctance torque Tr. Assuming that the current phase has an electrical degree of 0° when the electromagnet is positioned at a center between each permanent magnet and at a center of the projecting pole, the magnet torque Tm becomes a maximum value when the current phase is 0°, and the reluctance torque Tr becomes a maximum value when the current phase is 45°. Therefore, the maximum value of the total torque Tt is obtained when the current phase is at a value θ1 (30° for example) between 0 and 45°, where the magnet torque Tm is not the maximum value.
However, the above conventional motor is such that the maximum value of the total torque Tt is obtained while the magnet torque Tm is not at the maximum value. For this reason, the magnet torque Tm cannot be sufficiently utilized. Therefore, generating the total torque Tt of a required value requires a larger permanent magnet. Consequently, the size of the motor is increased.
Meanwhile, a motor driven at a high speed increases a resistance which is indicated as a product of a motor angular speed ω and a q-axis inductance Lq. Therefore, in order to decrease inductance for the projecting pole, that is, a product of the q-axis inductance Lq and a q-axis current iq, the current phase (mentioned above) must be advanced toward 90°, for example, to a value θ2 (such as 70°). In this case, the quantity of magnetic flux passing through the projecting pole is decreased, thus a torque constant of the motor becomes smaller. Therefore, the magnet torque Tm and the reluctance torque Tr are reduced, thereby reducing the total torque Tt as well. As a result, the output generated by the motor becomes smaller.